This invention relates to a master cylinder whose seal means for sealing the outer periphery of a piston is a double structured seal ring (namely, a double seal ring) including an inner peripheral seal ring and an outer peripheral seal ring situated on the outer periphery of the internal seal ring. In other words, this invention relates to a technique which can broadly be applied to a master cylinder used for a brake or a clutch, and more specifically to a master cylinder, in which an opening portion of a relief port formed in a cylinder body is blocked or interrupted by a seal ring in accordance with motion of the piston. More particularly, this invention relates to a technique suited to be applied to a tandem type master cylinder used for an anti-skid brake system.
The idea itself, for sealing the outer periphery of a piston with a double seal ring consisting of an inner peripheral seal ring and an outer peripheral seal ring is previously known. For example, Japanese Patent Laid-Open Publication No. 219263/1994 or U.S. Pat. No. 5,168,791 showing a hydraulic pressure type booster device, discloses a technique, in which, in order to seal the outer periphery of the servo piston, an O-ring and a resin ring are fitted in a groove formed in the outer periphery of the servo piston, the former on the inner periphery side and the latter on the outer periphery side. The inner peripheral seal ring made of rubber or the like is for sealing a space between the piston and the cylinder body, and a space between the outer peripheral seal ring and the cylinder body under the effect of its own elastic deformation. Moreover, the outer peripheral seal ring is subjected to an elastic force of the inner peripheral seal ring to seal a space between the inner periphery of the cylinder body and the piston. Since the outer peripheral seal ring is smaller in sliding resistance than the inner peripheral O-ring made of rubber, a smooth movement of the moving piston is facilitated.
Paying attention to the fact that the double seal ring is hardly susceptible to biting by seal (namely, a phenomenon in which the lip portion of the seal ring is bited and damaged by an opening of the relief port formed in the cylinder body), the present inventors recently proposed a technique in which the double seal ring is applied to a master cylinder. As one of such proposals, Japanese Patent Application No. 321190/1995 filed on Nov. 16, 1995 (laid open for public inspection on May 27, 1997) under 136639/1997) discloses a technique, in which a seal ring of a double seal ring structure is formed on an outer peripheral surface of a piston, a communication path for communicating a working fluid reservoir with a hydraulic pressure chamber is formed within a piston in order to supply a supplemental fluid to the hydraulic pressure chamber, and a supplementary valve is disposed on the communication path. Similarly, Japanese Patent Application No. 198487/1996 filed on Jul. 9, 1996 discloses another technique, in which a seal ring of a double seal ring structure is formed on the outer periphery of the piston, and a small piston is movably disposed within the piston, so that when the pressure in the hydraulic pressure chamber becomes negative and a pressure difference occurs between the atmospheric pressure on the side of the working fluid reservoir and the pressure in the hydraulic pressure chamber, the small piston is caused to move by this pressure difference, thus reducing the capacity of the hydraulic pressure chamber. A capacity reducing means including the small piston and a supplementary means including the supplementary valve are for supplementing a cup type seal ring having no supplementary feeding function.
By the way, in those preceding techniques, either well known or not, a groove for the seal ring to be fitted therein is formed in the outer periphery of the piston, and both the O-ring and the resin ring constituting the double sealing are arranged in the same groove with their enlarged diameter portions climbed over adjacent land portions of the piston. When the double seal ring is arranged in the groove formed in the outer periphery of the piston, the O-ring on the inner periphery is easily elastically deformed, and such deformed configuration of the inner peripheral O-ring is rapidly returned to its original configuration. However, the outer peripheral resin ring, which is harder than the O-ring, is difficult to be elastically deformed. Moreover, the outer peripheral resin ring, once deformed in configuration, is not easily returned to its original configuration. For this reason, a special tool is required for fitting the resin ring. Moreover, it is necessary to leave the resin ring as it is for quite a long time until the resin ring is restored to its original state. Even if returned to its original state, the resin ring, which is not easily elastically deformed, is not returned to its original state correctly in a strict sense. This can result in insufficient sealing performance and deteriorated sliding performance. In the conventional master cylinder using a cup type seal ring, the use of a return spring retainer as means for supporting the cup type seal ring is well known (U.S. Pat. No. 4,437,310).
Further, in the master cylinder including a double seal ring, aside from the problem pertaining to the assembly of such a resin ring, there is encountered with such an inconvenience that when the land portion of the piston is faced with an opening of a relief port on the side of the cylinder body immediately before the piston reaches its returned position, a space formed between the inner wall of the cylinder body and the land portion acts as a throttling for interrupting the return of the fluid towards the reservoir because of the following reason. In order to reliably support the double seal ring, or in order to maintain a mechanical strength of the piston in the vicinity of the attachment groove, an axial distance of the land portion adjacent the attachment groove must have a prescribed length. On the other hand, in order to reduce operating play (namely, invalid stroke), the end of the double seal ring is located as closer to the opening of the relief port as possible. In view of such a design concept, the land portion of the piston must be faced with the opening of the relief port, and thus, the afore-mentioned throttling is resulted.